The present invention relates to a moisture vapor barrier, and more particularly a moisture vapor barrier for paper and paperboard.
Fibrous substrates such as paper are widely used in packaging operations. Such fibrous substrates can be in sheet and roll form, and can be paper, paperboard, preformed paper containers, fabric and other fiber-based materials. Typically, fibrous substrates are subject to poor resistance to water vapor, gases, oil, solvents and greases. To improve the water vapor barrier resistance the fibrous substrates have been coated with a wide variety of compositions particularly when the substrates are used for food packaging.
Various coatings of this type are described in U.S. Pat. No. 3,632,424 to Graham et al. Of particular interest is the use of a wax or waxes incorporated into a latex and applied to a fibrous substrate as a coating. The physical phenomenon accounting for the good moisture vapor barrier properties of wax-coated paper is that the wax tends to migrate to the film surface to form a continuous wax layer. In laminating operations, however, this barrier layer can be interrupted; for example, such layers are susceptible to creases. Creases substantially reduce the water vapor resistance properties.
Additionally, the presence of wax makes recycling (repulping) of the fibrous substrate difficult. A conventional repulping operation typically comprises mixing water and the fibrous substrate, and the fibers are separated via mechanical action. The slurry may then be passed through screens and centrifugal cleaners to remove non-fibrous contaminants such as glass, metal, plastic, dirt, and other unwanted solid materials. In such repulping operations, the wax will separate and float to the surface of the repulping vessel. After waste paper is repulped, it is formed into paper or paperboard on the moving wire of a fourdrinier machine or on the cylinders of a cylinder machine, pressed and dried to remove the remaining water. If the separated wax is inadvertently incorporated into a paper sheet, the wax will appear as a grease spot, and coating and printing can be adversely affected.
It is therefore an object of the invention to provide a fibrous substrate having the moisture vapor layer applied thereto which maintains vapor resistance properties even if the fibrous substrate is creased.
Still another object of the present invention is to provide a moisture vapor barrier which obviates the problem associated with wax during repulping operations.
Another object of the invention is to provide a fibrous substrate having the moisture vapor layer applied thereto that can be printed, coated and bonded (e.g., using adhesive or glue) to another similar or dissimilar substrate.